The technical field of this invention is Raman spectroscopy and, in particular, probes for analyzing samples in Raman spectroscopy systems.
It is known in the art that the chemical analysis of materials can be determined by optical spectrum analysis of samples. The optical analysis can be based on near or mid infrared (IR) spectroscopy. However, many liquids, especially aqueous solutions, do not exhibit simple IR spectrums. Moreover, mid IR spectroscopy is not well suited for fiber optic data transmission.
Raman spectroscopy provides an alternative approach to analyzing samples that is often better suited for aqueous and other liquid environments and, perhaps more generally, for applications where it is desirable to sample in-situ. Raman spectroscopy is an analytical technique that uses light scattering to identify and quantify molecules. When light of a single wavelength (monochromatic) interacts with a molecule, the light scattered by the molecule contains small amounts of light with wavelengths different from the incident light. The wavelengths present in the scattered light are characteristic of the structure of the molecule, and the intensity of this light is dependent on the concentration of these molecules. Thus, the identities and concentrations of various molecules in a substance can be determined by illuminating the substance with monochromatic light and then measuring the individual wavelengths and their intensities in the scattered light.
Until recently, the major drawback of Raman spectroscopy has been its expense relative to mid IR and near IR spectroscopy systems. A significant component of that expense is the monochromatic laser system required to produce quality, high-resolution spectra. Even using a laser diode as the scattering source, the laser remains as one of the major expenses in developing cost-effective Raman systems.
U.S. Pat. No. 5,139,334 issued to Clarke, and incorporated herein by reference, teaches a low resolution Raman spectral analysis system for determining properties related to the hydrocarbon content of fluids, in particular, the octane rating of gasoline. Different fuel properties are determined by a method that compares Raman-scattered light intensities over different wavelength ranges. Because the system uses low resolution analytical techniques, the constraints on the laser excitation source are significantly relaxed. This improvement, together with the continued miniaturization of electronic components has made lower cost, portable Raman spectrometers practical.
A variety of spectroscopic probes suitable for Raman spectroscopy are known in the art. Such probes are available, for example, from InPhotonics, Inc. of Norwood, Massachusetts. U.S. Pat. No. 5,112,127 is illustrative of such probe assemblies. Typically, the probe head is cylindrical with at least two fiber optic channels, one to carry excitation radiation to the sample and another to carry scattered Raman radiation back to a detector. The input and output light paths are arranged in a collinear fashion within the probe head and require precise alignment of various optical components within a small cylindrical space.
The size and shape of conventional probes typically preclude incorporation of any safety features, such as shut-off switches, into the probe head, itself. Moreover, in harsh process environments, conventional probes are also susceptible to damage by high temperatures, pressures and/or chemical solvents.
There exists a need for better Raman spectroscopic systems suitable for sampling liquids, tablets, powders and the like. Moreover, there exists a need for better probe designs for use with low-resolution Raman spectroscopic systems, especially with portable or handheld systems.